The object of this invention is to provide a press whereby pressure can be continuously applied to a substrate such as plywood, wood particle molding compound, laminates and the like while the substrate is simultaneously being moved through the press.
The importance of such a press is apparent when consideration is given to the relation of time and temperature in the curing of the adhesive used; for example, in the production of particle board, say 3/4 inch thick, a pressing time of 8 to 10 minutes may be required in the hot press in order to allow time for the center of the strip to reach curing temperature, it being necessary for heat to be transferred from the surfaces of the press platens through 3/8 inch of molding compound.
If, however, the molding compound could be quickly brought to press temperature outside the press and then injected into the press in a matter of a few seconds the press time need be only one minute or less since, at the press temperature employed, the resin can be cured in this short period.
However, with conventional presses in which the molding compound is spread on a tray or caul and then injected into the open press, the time involved in preparing the mat on the caul and loafing and unloading the press is so great that the molding compound must be handled at low temperature in order to prevent precure of the resin or binder before pressure can be applied. Thus with an intermittent pressing operation it is imperative to use cold molding compound which, in turn, requires that the molding compound be raised to curing temperature in the press. This accounts for long press cycles and requires large equipment to achieve substantial production rate.
If, however, the pressing operation could be made continuous so that the mat of molding compound could be laid in the throat of the press continuously and then moved immediately into the press such that the elapsed time between laying of the mat and application of full pressure (with simultaneous reduction to final thickness) could be accomplished in say 10 to 15 seconds then it would become possible to rapidly preheat the molding compound close to curing temperature and draw it into the high pressure zone in a matter of seconds after preheating without danger of precuring. The elapsed time between mat formation and completion of this finished strip would then be only the nominal cure time of the resin system which might be say one minute, since the press platens need to deliver very little heat to the curing mass. Thus the platen areas required for a given throughput of material would be reduced by the ratio of press time of conventional versus a continuous system. This reduction can be of the order of one-third to one-sixth depending on strip thickness. In fact, with a continuous system, using a preheated substrate, very substantial thickness may be cured in a very short time.